A leak check blind, according to the present invention, allows an in-line valve seat to be tested without removal of the valve from the line. The leak check blind is inserted in the flow line upstream from the valve to be tested. The blind is connected to a pressurized fluid supply for injecting a flow (pressure) to the valve and external flow measuring devices are provided for monitoring any flow of the test fluid. Any test fluid flow is indicative of a leaking in-line valve. If the detected leak is outside prescribed limits, the valve is then removed from the line and either repaired or replaced with a new valve. If the leak is within acceptable limits, the valve is simply reconnected and put back in service.